not on the 58, it's on the east coast - not far from where you are. On the 329 heading north out of Ishikawa towards Kin. Here's a map link - you can see the yard with lots of boats in just behind the 329.

You will find all kinds of stuff there. The other specifically marine supply store on Island is all the way down in Naha at Tomari port (where the ferries over to the Kerama Islands leave from). There used to be a place on 58 in Ginowan called "Offland" who stocked Hobie stuff but I don't know if he's still around. I tried calling them a while back and could get no answer.